1. Field of the Invention
The present invention relates to an optical disc drive and method of controlling the focal position and is adapted to be applied, for example, to an optical disc drive for recording a hologram onto an optical disc.
2. Description of the Related Art
Heretofore, optical disc drives for reading information by directing a light beam at an optical disc (such as a CD (compact disc), DVD (digital versatile disc), or Blu-ray disc (trademark registered, hereinafter abbreviated BD)) and reading the reflected light have enjoyed wide acceptance.
Furthermore, in such a related-art optical disc drive, the reflectivity on the optical disc is locally varied by illuminating the disc with a light beam, thus recording information.
It is known that the size of the optical spot formed on this optical disc is roughly given by λ/NA (where λ is the wavelength of the light beam and NA is the numerical aperture) and that the resolution is in proportion to this value. For example, details of the BD technology capable of recording about 25 GB of data onto an optical disc having a diameter of 120 mm are described in Y. Kasami, Y. Kuroda, K. Seo, O. Kawakubo, S. Takagawa, M. Ono, and M. Yamada, Jpn. J. Appl. Phys. 39, 756 (2000) (non-patent reference 1).
Various kinds of information such as various kinds of multimedia-rich contents (e.g., music contents and video contents) and various kinds of data for computers are recorded on optical discs. Especially, in recent years, the amounts of information have increased because of improved resolution of videos and improved sound quality of music contents. Furthermore, increase in the number of contents recorded on one optical disc tends to be required. Therefore, there is a demand for a further increase in the storage capacity of the optical disc.
Accordingly, a technique for increasing the recording capacity of one optical disc by stacking plural recording layers within the single disc has also been proposed (see, for example, I. Ichimura et al., Technical Digest of ISOM, '04, p. 52, Oct. 11-15, 2005, Jeju, Korea (non-patent reference 2)).
On the other hand, an optical disc drive using holography has been proposed as a technique for recording information on an optical disc (see, for example, R. R. McLeod et al., “Microholographic multilayer optical disk data storage,” Appl. Opt., Vol. 44, 2005, p. 3197 (non-patent reference 3)).
For example, as shown in FIG. 1, an optical disc drive, indicated by reference numeral 1, uses an optical disc 8 made of photopolymer whose refractive index varies with the intensity of the light impinging on the disc. A light beam from an optical head 7 is once focused onto the disc 8. Then, the beam is again focused at the same focal position from the reverse direction using a reflector 9 mounted on the rear side (lower side as viewed in FIG. 1) of the optical disc 8.
In the optical disc drive 1, a light beam made of laser light is emitted from a laser 2 and the optical wave is modulated by an acoustooptic modulator 3. The beam is then converted into collimated light by a collimator lens 4. Subsequently, the light beam is transmitted through a polarizing beam splitter 5, and is converted from linear polarization to circular polarization by a ¼ wave plate 6. Then, the beam is made to hit the optical head 7.
The optical head 7 is designed to be capable of recording and reading information. The head reflects the light beam by means of a mirror 7A. The beam is condensed by an objective lens 7B and directed at the optical disc 8 rotated by a spindle motor (not shown).
At this time, the light beam is once brought to a focus inside the optical disc 8 and then reflected by the reflector 9 disposed on the rear side of the optical disc 8. The beam is focused at the same focal point inside the optical disc 8 from the rear side of the disc 8. The reflector 9 is made up of a condenser lens 9A, a shutter 9B, a condenser lens 9C, and a reflective mirror 9D.
As a result, as shown in FIG. 2A, stationary waves are produced at the focal position of the light beam, resulting in a recording mark RM made of a hologram of a small light spot size. As a whole, the mark assumes a form obtained by bonding together two cones at their bottoms. Thus, the recording mark RM is recorded as a piece of information.
When the recording mark RM is recorded plurally inside the optical disc 8, the optical disc drive 1 rotates the disc 8 and arranges the recording marks RM along coaxial or spiral tracks, thus forming one mark recording layer. Furthermore, the recording marks RM can be recorded in such a way that plural mark recording layers are stacked by adjusting the focal position of the light beam.
Consequently, the optical disc 8 has a multilayer structure having plural mark recording layers therein. For example, as shown in FIG. 2B, in the optical disc 8, the distance p1 (mark pitch) between the recording marks RM is 1.5 μm. The distance p2 (track pitch) between the adjacent tracks is 2 μm. The distance p3 between the adjacent layers is 22.5 μm.
In the optical disc drive 1, when information is read from the disc 8 on which the recording marks RM have been recorded, the shutter 9B of the reflector 9 is closed to prevent the light beam from being emitted from the rear side of the optical disc 8.
At this time, the optical disc drive 1 directs the light beam at any one of the recording marks RM within the optical disc 8 by the optical head 7. The readout light beam produced from the recording mark RM is made to hit the optical head 7. The readout light beam is converted from circular polarization into linear polarization by the ¼ wave plate 6 and reflected by the polarizing beam splitter 5. The readout light beam is condensed by the condenser lens 10 and made to hit the photodetector 12 via the pinhole 11.
At this time, the optical disc drive 1 detects the amount of light of the readout light beam by the photodetector 12 and reads out the information based on the result of the detection.
Furthermore, an optical disc drive using different kinds of light beams between the position control of the objective lens and recording/reading of information has also been proposed (see, for example, S-K Park, T. D. Milster, T. M. Miller, J. Buts and W. Bletscher, Jpn. J. Appl. Phys., Vol. 44 (2005) pp. 3442-3444 (non-patent reference 4)).
For example, as shown in FIG. 3, an optical disc drive 15 emits a position-controlling light beam L1 to an optical disc 18 via a beam splitter 16 and an objective lens 17.
In addition, the optical disc drive 15 controls the position. That is, the drive detects the returning light that is reflection of the position-controlling light beam L1 at the reflective surface 18A of the optical disc 18, and controls the focus of the objective lens 17 and the tracking according to the results of the detection. In this way, the position-controlling light beam L1 is brought to a focus onto a desired track on the reflective surface 18A.
Under this condition, in the optical disc drive 15, a recording/reading (write/read) light beam L2 different from the position-controlling light beam L1 is reflected by the beam splitter 16 and brought to a focus onto a recording layer 182 of the disc 18 via the objective lens 17 whose position is controlled. Thus, information (such as recording marks RM) is recorded or read out.